1. Field
The embodiment relates to shunt regulators and electronic apparatuses, and particularly to a shunt regulator which controls supply voltage within a given range and an electronic apparatus which operates on power supplied by radio.
2. Description of Related Art
IC cards and ID chips which do not contain a battery as a power source receive radio energy emitted from a reader-writer and obtain power therefrom. The power received by these IC cards and the like changes greatly with the distance from the reader-writer, and the supply voltage also changes greatly. A great increase in supply voltage would result in damage to transistors and the like in the IC card. The IC cards and the like use a shunt regulator or a clamp circuit in order to suppress the great increase in supply voltage (see Japanese Unexamined Patent Application Publication No. 2003-296683 and Japanese Unexamined Patent Application Publication No. 2001-217689, for instance).
FIG. 10 shows a circuit diagram of a conventional shunt regulator. As shown in the diagram, the shunt regulator includes a PMOS transistor M101, resistors R101 and R102, and a capacitor C101.
Power supplied from the reader-writer is rectified by a rectifier and supplied to a load 101. The shunt regulator controls the power (voltage Vdd) rectified by the rectifier within a given range. To be more specific, if a current Iin supplied to the load 101 is excessive, the shunt regulator turns on the transistor M101 to pass a bypass current Ibp and prevents the voltage Vdd from increasing. The bypass current Ibp is designed to be sufficiently small in relation to a current Icons flowing through the load 101 such that, if the current Iin supplied to the load 101 is small and brings the voltage Vdd to the lower limit, the lower limit is obtained with a smaller current Iin.
FIG. 11 is a view illustrating an example of operation of the shunt regulator shown in FIG. 10. As shown in the figure, when the current Iin supplied to the load 101 becomes the current Icons, a voltage Vddmin, which is the lower limit of the voltage Vdd, is obtained. When an increase in the current Iin increases the voltage Vdd, the shunt regulator passes the bypass current Ibp through the transistor M101 to prevent the voltage Vdd from increasing. The shunt regulator controls the voltage Vdd within the range of the voltage Vddmin to a voltage Vddmax by passing the bypass current Ibp to supply an appropriate supply voltage to the load 101. If the current Iin exceeds the current Iin max, the voltage Vdd would exceed the upper-limit voltage Vddmax, disabling the normal operation of the load 101. Otherwise, there would be a possibility that the voltage exceeding the withstand voltage would damage the load 101.
The shunt regulator shown in FIG. 10 passes the bypass current Ibp given by the following expression (1).
                    Ibp        =                              β            2                    ⁢                                    (                                                Vdd                  ⁢                                                            R                      ⁢                                                                                          ⁢                      101                                                                                      R                        ⁢                                                                                                  ⁢                        101                                            +                                              R                        ⁢                                                                                                  ⁢                        102                                                                                            -                Vthp                            )                        2                                              (        1        )            
In the expression (1), β is a parameter determined by the characteristics of the transistor M101, such as the gate width and the mobility of electrons, and Vthp is the threshold voltage at which the transistor M101 turns on.
The expression (1) tells that the bypass current Ibp varies with the characteristics of the transistor M101 or the threshold voltage Vthp. Accordingly, the variation in the transistor M101 would affect the bypass current Ibp and change the range of the voltage Vdd.
FIG. 12 is a view showing the relationship between the voltage and the bypass current, affected by the variation in the transistor. A straight line L101 shown in the figure expresses the desired relationship between the voltage Vdd and the bypass current Ibp. The bypass current Ibp should be 0 at the lower-limit voltage Vddmin, and the bypass current Ipb should become the current Iin max at the higher-limit voltage Vddmax.
If the threshold voltage Vthp of the transistor M101 varies, the straight line L101 will slide to the left or right as indicated by an arrowed line P101 in the figure. The variation in β will also change the inclination of the straight line L101, as indicated by arrowed lines P102. Consequently, the variation in the transistor M101 may make it impossible to keep the voltage Vdd within a desired range.